text: Fluid and electrolyte balance, in which fluid balance and electrolyte balance are intertwined homeostatically, is necessary to health in all organisms. It includes reference ranges for cation concentrations of biometals, which in reference to human medicine and veterinary medicine principally includes those for blood serum ion concentrations in humans and in livestock and pets. Derangements in such fluid and electrolyte balance most often occur in the contexts of dehydration, overexertion, and diarrhea, but they also occur in cancers (most especially in paraneoplastic syndromes), parasitism, inborn errors of metabolism, and several other contexts. Some medical specialties deal especially frequently with electrolyte derangements, including internal medicine and endocrinology (especially in chronic conditions) and intensive care medicine (in severe acute conditions).